1. Field of the invention
The present invention relates to the field of inorganic complex materials technology, more specifically, the invention relates to rare earth alumina powders retaining a high thermal stability, manufacturing methods and applications. The material of the present invention is especially useful for three-way catalyst supports in mobile engine exhaust converters.
2. Description of the Prior Art
The pollution of the atmosphere mainly comes from mobile engine exhaust consisting of carbon monoxide (CO), all kinds of hydrogen carbon compounds (HC) and nitrogen oxygen compounds (NOx). The three-way catalyst can change most of the pollution compounds into innocuous carbon dioxide, CO2, water H2O and Nitrogen N2. Therefore it is called an environmental conservation catalyst. The mobile engine exhaust filter is composed of noble metals for catalytic activity (platinum, rhodium and palladium), oxygen storage materials and catalyst support alumina as well as the cordierite honeycomb ceramic that supports all of the powders. As we know, the efficiency of a catalyst is generally related to the interface area of catalyst and reactive compound. The greater the surface (contact) area is, the higher the efficiency. The basic function of a catalyst support is to enable the catalyst particles to retain a separated status as much as possible and therefore enable the active catalyst particles adequately contact the reactive compounds to promote the chemical reactions. In the high temperature environment of engine exhaust, the carrier material supporting the catalytic reactive noble metal must have superior thermal resistance capability. Thus, at high temperature, especially at 1200° C., the carrier material can still retain a high surface area.
The common alumina support has poor thermal resistance capability characterized by a high temperature phase formation, collapse of most pores and a sudden drop of surface area below 10 m2/g at 1200° C.
In the 1970's, some US patents disclosed the methods of making catalyst support materials, i.e., impregnating transition alumina with soluble rare earth salts in solution followed by the separation of the liquid from the solids and calcinations of the solids in order to improve alumina thermal stability.
In 1977, U.S. Pat. No. 4,061,594 disclosed a method of making alumina of high resistance to high temperature by impregnating solutions of rare earth nitrates, chlorides and acetates, in which the rare earth elements included La, Pr, Nd and Th. But the effect is not obvious because the aged SA at up to 1200° C. quickly decreased as the calcining time was prolonged. For example, the specific surface area after calcination at 1200° C. for 4 hours is not greater than 32 m2/g.
In 1996, U.S. Pat. No. 5,718,879 disclosed another similar method of making heat-stable alumina particulates having a specific surface area of greater than 40 m2/g after calcination at 1200° C. for 4 hours by ripening/rehydrating an alumina powder at least partially into a boehmite/pseudoboehmite state in the presence of a stabilizing amount of at least one lanthanum compound, e.g. lanthanum nitrate and/or at least one neodymium compound followed by liquid solid separation, drying and calcination. But after calcination at 1200° C. for 24 hours, the aged SA is only 28 m2/g.
U.S. Pat. No. 4,722,920 disclosed a method of making an alumina catalyst support stable at high temperatures comprising a transformative alumina having a purity not less than 99.95% and particle size not greater than 0.5 microns impregnated with lanthanum in amount of 1.5 to 6.0 wt % based on the weight of the alumina. The alumina catalyst support has a surface area of at least 60 m2/g after heating at 1200° C. for 5 hours for a transformative alumina having a purity of 99.99%, but if the purity is lowered to 99.95% the aged SA is decreased by at least 12%.
SASOL's commercial product, PURALOX-SCFa-140L3 of high-purity (Na2O<20 ppm) was prepared from hydrolysis of aluminum alkoxide. The alumina catalyst support containing 4% La2O3 has an SA of greater than 40 m2/g after heating at 1200° C. for 24 hours. The production cost, however, is high for this process.
In industrial production, the precipitation step can be carried out in a tank reactor with a stirrer and temperature controller. The process can be batch or continuous. In general, a continuous process has a higher stability and consistency as well as a higher efficiency than a batch process. The uniformity of the former, however, is lower than that of the latter, which lowers the properties of the product.